Biocompatible implant materials made of metal have conventionally been used. In recent years, however, ceramics are attracting attention from the standpoint of biocompatibility, and ceramic implant materials have been put to practical use. In particular, calcium phosphate compounds have excellent biocompatibility, and sintered bodies obtained therefrom are known as a material which chemically bonds to a bone or is replaced by a bone.
The present inventors have already proposed in JP-B-60-50744 (the term "JP-B" as used herein means an "examined Japanese patent publication") a process for producing a calcium phosphate sintered body having good biocompatibility and high strength which process comprises incorporating an alkaline earth metal oxide/phosphoric acid frit into a powder consisting mainly of a calcium phosphate having a calcium atom/phosphorus atom ratio of from 1.4 to 1.75, followed by firing, the amount of the frit being 0.5 to 15% by weight based on the amount of the resulting calcium phosphate sintered body. This process succeeded in producing biocompatible implant materials having excellent biocompatibility and high mechanical strength. These implant materials, when implanted in living bodies, chemically bonded to bone tissues and did not readily fracture due to their high strength, giving satisfactory results. However, bone growth on these implant materials was insufficient because bone tissues did not penetrate into the implant materials. As a result, much time was required for bone tissues to adhere to the implant materials.
On the other hand, porous implant materials have excellent biocompatibility although inferior in strength to dense materials. In particular, porous implant materials having pores as large as from tens to hundreds of micrometers are known as a material into which bone tissues readily penetrate. Known methods for producing porous objects include the technique of incorporating a combustible pore-forming material, e.g., an organic substance or carbon, into a raw material and the technique of incorporating a foaming agent or the like. These techniques have long been employed for the production of bricks and other products. If these techniques are applied to a calcium phosphate material, porous objects for implantation may be produced.
However, the porous biocompatible implant materials produced by such conventional techniques frequently have shown insufficient bone growth because the penetration of bone tissues into pores thereof does not occur, even though the implant materials are a porous calcium phosphate object having pores as large as, e.g., about 100 .mu.m.
Such conventional porous objects have had another drawback because of the significantly low strength thereof. Specifically, touching or rubbing with a finger results in falling of particles or breakage, so that the porous-objects have very poor handleability in bone compensation surgery.